Apparatus for processing of semiconductor materials by ion implantation has been continuously refined in the ability to produce uniform deposition of ion dose over the surface of a semiconductor wafer. Known effects which would alter uniform dose density are compensated in systems which scan the incident ion beam in accord with a waveform selected to effect the desired compensation by homogenizing the scan trace along the path of such trace. Such a system is described in U.S. Pat. No. 4,283,631, to Turner. Another approach ignores inhomogeneity at the level of the individual trace and compensates known nonuniformity (or more generally, deviation from a desired dose distribution) by controlling the density of traces distribution as a function of location on the semiconductor surface. This system is described in U.S. Pat. No. 4,449,051. Optimum deployment of these examples of the prior art each assume knowledge of the actual distribution of which compensation is desired.
Measurement of dose nonuniformity is not unambiguously established on-line in the prior art. Typically, a wafer is positioned in the target plane of a Faraday cage charge collection enclosure with a number (typically 4) of auxilliary Faraday cups equidistantly disposed along the periphery of the workpiece to monitor the beam current during an overscan portion of the scan trace for those traces which intercept these auxilliary Faraday cups. It is apparent that a discrete set of charge integration measurements from peripheral Faraday cups does not establish the does distribution which actually obtains over the wafer surface. This (unmeasured) distribution is assumed in the prior art on the basis of these off-wafer measurements and is properly regarded as an interpolation. It is also apparent that monitoring an overscan requires that a substantial portion of the trace be employed for no other purpose than to serve the monitoring function. A portion of the scan is therefore unproductive with respect to the primary purpose of material processing, resulting in a deadtime interval. Prior art approaches to differential dose uniformity monitoring are described by Hammer and Michel, J. Appl. Phys., v.47, pp 2161-2164 and by Natsuoki, et al, Rev. Sci. Inst. v. 49, pp. 1300-1304.
Accordingly, it is an object of the invention to secure an online, differential non-interpolated measurement of dose distribution during irradiation of a workpiece.
It is another object of the invention to maximize the duty cycle for a beam scanning system by eliminating measurements executed off the workpiece.
It is still another object to provide a closed loop system for conforming the actual dose distribution from an irradiation flux impinging a workpiece to a predetermined dose distribution.